Quantum-Limited Magnetometry with Cavity Magnon Polaritons

Researchers have developed two novel types of spin-magnetometers using photon-magnon hybrid systems (PMHSs) that achieve remarkable magnetic sensitivity by leveraging cavity magnon polaritons (CMPs). These devices translate the quantum-limited precision of microwave detectors to magnetic field measurements, opening new possibilities in quantum sensing and fundamental physics exploration.

Photon-magnon hybrid systems are created by coupling the electron spin resonance of magnetic materials like Yttrium Iron Garnet (YIG) spheres with microwave cavity modes. When strongly coupled, these systems produce quasiparticles called cavity magnon polaritons, which mix magnons (quantized spin excitations) and photons. This mixing enables efficient conversion between magnetic and electromagnetic excitations, allowing magnetic information to be extracted by monitoring microwave photons.

The research team implemented two distinct magnetometer designs. The transverse spin-magnetometer (TSM) detects oscillating magnetic fields perpendicular to a static field by measuring power absorption and re-emission in hybrid modes. Using a system with ten YIG spheres in a cylindrical cavity cooled to milli-Kelvin temperatures and read out with a Josephson Parametric Converter, they achieved a sensitivity of 0.9 × 10^(-18) T/√Hz. This device was used to search for dark matter axions, setting a limit of 5.5 × 10^(-19) T on axion-induced fields.

The longitudinal spin-magnetometer (LSM) operates with oscillating fields parallel to the static field, detecting sidebands in the transmission spectrum caused by frequency modulation of hybrid modes. A room-temperature prototype demonstrated a sensitivity of 2.0 ± 0.4 pT/√Hz, competitive with state-of-the-art magnetometers like SQUIDs. The LSM's sensitivity benefits from high pump power and quality factors, and its size-independent nature allows for miniaturization.

Both devices overcome limitations of traditional techniques by operating in the strong coupling regime where the interaction strength exceeds damping rates, ensuring efficient signal transduction. The CMPs' immunity to radiation damping at GHz frequencies further enhances sensitivity. Future improvements could involve broadband traveling wave parametric amplifiers and single quantum counters to surpass the standard quantum limit.

These advancements position PMHS-based magnetometers as powerful tools for precision measurements in quantum information science, fundamental physics tests, and potentially broader technological applications where high magnetic sensitivity is required.